Over the past few decades, we have spent billions of dollars in efforts to develop an effective vaccine or microbicide aimed at blocking a very specific event in the early stages of HIV transmission. Specifically, many therapies seek to block the initial infection of a single or small number of target cells in the course of sexual transmission. This initial event(s) ultimately leads to the production of hundreds of billions of viral particles that destroy the immune system leading to the development of AIDS and ultimately death. Naturally, the blockade of this initial event(s) would abort the entire process and prevent disease. It is unfortunate that after almost 30 years of studying HIV, we still know very little about this key event. To move the field forward, it is critical that we know which cell type(s) are infected and where are the preferential sites of initial transmission. Towards this goal, we have recently developed a system that allows the precise identification of the first exposed target cells in the genital tract. This system utilizes a SIV-based gene therapy vector, which is pseudotyped with an HIV envelope. Expression of a mCherry florescent protein by the transduced cell reveals which cell(s) was infected by the vector. No viral proteins are expressed by the vector system, thus it cannot spread to another cell. Preliminary results suggest that the system can be utilized to identify transduced target cells after vaginal exposure in female rhesus macaques. We believe that the rhesus macaque is currently the most widely used and perhaps most relevant model available to gain insights into what may be happening in humans. We will utilize this system to study the earliest events of transmission after vaginal exposure in a living model. These studies will provide novel insights into the initial targets of infection after sexual exposure and the preferred location of the transmission event. With this knowledge, it may be possible to optimize prevention strategies to protect the most susceptible cells in the most susceptible locations bring the goal of a successful vaccine or microbicide one step closer to being realized.